Various image enlarging methods have been suggested. In particular, a method that executes a filter process on an image after upsampling the image (or at the same time as an uplink) is well known. As a method that calculates increased brightness of a pixel by an interpolation to enlarge an image, a bilinear interpolation and a bicubic interpolation are known (for example, refer to Non-Patent Document 1). FIG. 27 illustrates an example of an image where a low-resolution image is upsampled with the double magnification in each of longitudinal and transverse directions. FIG. 28 illustrates an example of an image where brightness of a pixel in an enlarged image is interpolated by a bi-cubic interpolation.
In these methods, an image after the enlargement may blur. This is because a high-frequency component of the image is lost with enlargement of the image. For this reason, it is general to apply a contour enhancement process in order to compensate for the lost high-frequency component and restore the original contour.
An example of an image processing apparatus to enhance the contour is described in Patent Document 1. FIG. 29 is a block diagram illustrating the image processing apparatus that is described in Patent Document 1. As illustrated in FIG. 29, the image processing apparatus that is described in Patent Document 1 includes an analysis circuit 902 that has a contour component extracting circuit 901, a signal level detecting circuit 902, and a contour detecting circuit 904, a parameter calculating circuit 903, and a contour correcting circuit 905.
The analysis circuit 920 receives an input video signal Si and an input source determination signal Ssel. The input source determination signal Ssel is a signal indicating a kind of the input video signal Si. For example, when the input video signal Si is a video signal based on an HDTV (High Definition Television) scheme, a level of the input video signal Si becomes an H level (high level). Meanwhile, when the input video signal Si is a video signal based on an NTSC (National Television System Committee) scheme, the level of the input video signal Si becomes an L level (low level).
In the image processing apparatus, the contour component extracting circuit 901 extracts a contour component (also called an “edge component” or a “contour signal”) Sa, which is a component corresponding to the contour in an image indicated by the input video signal Si, from the input video signal Si, according to the input source determination signal Ssel. The contour component extracting circuit 901 includes a high-pass filter. For example, the contour component extracting circuit 901 extracts a secondary differential component of the input video signal Si using a secondary differential filter and outputs the secondary differential component as the contour component Sa.
The signal level detecting circuit 902 detects a signal level of the input video signal Si and outputs a signal that has the detected signal level as a signal value.
The parameter calculating circuit 903 receives a first parameter αi, a second parameter βi, and a third parameter Di from the outside, and calculates an excessive enhancement correction threshold value α functioning as a first threshold value to suppress the contour from being excessively enhanced, a minute component enhancement threshold value β functioning as a second threshold value to enhance the contour of a minute image, and a contour detection threshold value D functioning as a third threshold value to detect the contour, based on the external parameters αi, βi, and Di.
The contour detecting circuit 904 compares the contour detection threshold value D and a value obtained by calculating a variation of the input video signal Si, and outputs a contour detection signal Sc indicating existence or non-existence of the contour, based on the comparison result.
The contour correcting circuit 905 uses the contour component Sa output from the contour component extracting circuit 901, the contour detection signal Sc output from the contour detecting circuit 904, the excessive enhancement correction threshold value α and the minute component enhancement threshold value β output from the parameter calculating circuit 903, and a coefficient γ externally given as a gain adjustment parameter, performs a signal process for performing contour enhancement on the input video signal Si, and generates an output video signal Sd as a signal process result.
In Non-Patent Document 1, an EM algorithm (Expectation Maximization algorithm) is described.
[Patent Document 1] Japanese Patent Application Laid-Open No. 2002-290773 (paragraphs 0041 to 0043, FIG. 1)
[Non-Patent Document 1] “Handbook of Image Analysis [Revised Edition]”, first edition, University of Tokyo Press, Sep. 10, 2004, p. 728-732, p. 1362-1367